Another fascinating feature of the ocean that is part of the topography are coral reefs. Coral reefs are found in the sea around the equator all around the world, but they are not formed by geologic processes. Reefs are hard-as-stone structures that have been built up over many thousands of years by tiny organisms called corals leaving behind their skeletal remains. Corals are related to jellyfish in that they are gelatinous creatures which come from the same family tree. They begin life as tiny, microscopic polyps that swim about and float freely in the seas, but eventually take root on the structure of a reef and spend the rest of their lives living under the protection of the reef. Their tiny bodies extract minerals from the sea water (calcium carbonate) and use them to form a hard shell around their bodies. The coral now has a safe haven from all the creatures in the sea that might like to eat it. After the coral dies, its protective shell remains behind, adding just a tiny bit of new “masonry” to the existing reef. This process is called accretion and, as you can imagine, is a very slow process. Over many thousands of years, the accumulation of these tiny bits of reef can build a very substantial structure.

Coral reefs are usually found in warmer, tropical seas surrounding the equatorial belt. Typically, corals will invade the seas around a sheltered volcanic island, taking root on the fresh rock below the surface. The Great Barrier Reef, off the south coast of Australia, is the largest coral reef in the world. It stretches for 1,600 miles (2600km) and covers over 133,000 square miles of ocean floor and is composed of 2,900 individual reefs. It is estimated that over 350 different species of coral live on the Barrier Reef and there are as many as 70 different habitats within the reef. Scientists estimate the reef is approximately 20,000 years old, but geologists who've extracted samples from deep within the reef have been dated at 500,000 years old. The Great Barrier Reef is so big it can be seen from outer space and is the largest structure in the world built by living organisms.